For organic electronic devices, for example organic light-emitting diode displays (OLEDs), the laser ablation process has been used to remove organic materials from unwanted performance sensitive areas of the device. For illustration purposes, for one technique the first steps in the fabrication of OLED devices are the deposition and patterning of the conductive members (e.g., electrodes, such as the anodes) and conductive leads. The anode is typically indium tin oxide (ITO), and the conductive leads may include a tri-layer sandwich including an adhesion layer, a low-resistivity conducting layer, and a protective capping layer. A typical OLED conductive lead structure is Cr/Cu/Cr. After forming the anode and conductive leads, the process involves forming a cathode-separation layer followed by formation of an organic layer, such as a buffer layer (also known as a charge transport layer) (BL) and an electro-luminescent layer (EL), over the entire surface of the substrate. In the current practice in the field, the substrate then goes into the laser ablation system, where the laser beam is focused onto areas that need to be cleared of the organic layer. These include the electrode-to-conductive lead electrical contact pads, bond pads, and the frame (sometimes called the rail) around the active area upon which glue is dispensed for an encapsulating lid, which are examples of the performance sensitive areas of an OLED.
The laser ablation process has serious drawbacks and is a weak link in the fabrication of OLED devices in large volumes. The laser ablation machine is expensive, its modules are difficult to maintain, and the machine as a whole is not easily interfaced with other process tools.
Furthermore, from a materials point of view, the laser ablation process is not well suited to the removal of the organic layer, as it often leaves behind a tough carbonaceous residue that is resistant to removal unless a gas, such as oxygen is added to aid in the formation of volatile oxide species. However, the use of oxygen is risky as the sensitive EL organic active layer is exposed to the atmosphere during the ablation process, which is usually pure nitrogen. The residue problem is mitigated somewhat by using a high laser fluence (measured in mJ/pulse), but the undesirable side effect of this is the cracking of the underlying leads structure. The reliability of the device is thus compromised.
To further complicate the process, the wavelengths of the excimer laser beam available are limited to a few specific values, and so the target organic materials (for example the BL and EL) are forced to have absorption bands in that same wavelength region. This removes an important degree of freedom, and the ablation efficiency is unavoidably compromised.
Towards the end of the ablation process, the conductive leads to which contact is to be made can experience the full brunt of the laser energy, and thus get partly ablated as well. In some cases, the absorption of the laser photons by the conductive leads may be higher than that of the organic layer, in which case the process becomes unusable.
Wet cleaning surfaces with solvents and materials introduces a variety of problems into the manufacturing process of electronic devices.
Thus, improved processes are needed to clean all types of organic materials from performance sensitive areas of an organic electronic device.